(1) Field of the Invention
The present invention relates to a flapping abutment mechanism for a lift assembly, to a rotorcraft rotor, and to a rotorcraft having such a rotor.
The invention thus lies in the technical field of rotorcraft rotors. More specifically, the invention lies in the field of abutments fitted to such rotors in order to limit flapping movement of the lift elements carried by the rotor.
(2) Description of Related Art
A rotorcraft usually has at least one rotor for providing the rotorcraft with at least some of its lift and possibly also with propulsion.
A rotor comprises at least two lift assemblies that are rotated by a drive system. Each lift assembly is fastened in particular to the drive system by at least one retention and mobility member.
For example, such a drive system may comprise a hub constrained to rotate with a rotary mast. Each lift assembly is then provided with a blade that is connected to the hub by means of a retaining and mobility member. A retaining and mobility member optionally includes a hinge referred to as a “spherical abutment”. Each spherical abutment may have one strength member fastened to the hub and another strength member fastened to a cuff, regardless of whether the cuff is or is not integral with a lift element of the blade.
Each blade may thus comprise a lift element fastened to a cuff or indeed a lift element having an integral cuff. The blades are thus substantially plane elongate elements that are carried at one end by a drive system.
The retaining and mobility members equipped each respective lift assembly at its root in such a manner as to allow the lift assemblies to be maneuvered by an operator. By way of example, mounting the lift assemblies movably on a drive system enables a pilot of the rotorcraft to act in flight to cause the pitch of the lift assemblies to vary collectively or cyclically in order to influence the behavior of the rotorcraft with respect to its lift and/or its propulsion.
By way of example, the ability of the lift assemblies to move relative to a drive system makes it possible for them to perform lead/lag movement, pitch movement, and up and down flapping movement. The concept of “up and down” should be considered relative to the direction in which the axis of rotation of the rotor extends. When the rotor is rotating at a nominal speed, the lift assemblies are spontaneously driven upwards under the effect of centrifugal force and of the lift from those lift assemblies. When the rotor is stationary, the lift assemblies are no longer subjected to any force other than gravity and they are therefore naturally moved downwards.
A rotor is thus commonly fitted with an “abutment mechanism” that limits the travel of lift assemblies, particularly in flapping.
In an embodiment, the abutment mechanism for each blade may comprise a bottom abutment member and a top abutment member forming obstacles to the individual flapping travel of its blade, respectively downwards and upwards. An abutment mechanism includes a bottom abutment track and a top abutment track, which tracks are arranged on a retention and mobility member. By way of example, a strength member of a spherical abutment may carry an abutment pad presenting an abutment track.
In the event of undesired flapping movement of the blade, the abutment track comes into contact against an abutment member, thereby limiting the flapping movement of the blade.
A particular function of the abutment mechanism is to limit the abutment movement of the blades when the rotor is stationary and while the rotor is being set into rotation, and more particularly in the presence of wind. The abutment mechanism can also limit the flapping of the blades during an operation of folding the blades. In particular in the presence of spherical abutments hinging a blade to the hub, the abutment mechanism tends to preserve the spherical abutment when the rotorcraft is on the ground by preventing flapping movement of a blade in the absence of centrifugal force which might lead to the component elements of the spherical abutment becoming delaminated.
While the rotor is in rotation, the top abutment members might possibly not constitute any obstacle to the freedom of the lift assemblies to move, in particular upwards. The abutment mechanism can then provide retraction means for retracting the top abutment members while the rotor is in operation.
The top abutment members are then mounted on the hub to be movable between two positions, namely between a position that is to be occupied below a threshold speed of rotation of the rotor, and a position that is to be occupied above that threshold speed of rotation.
In summary, an abutment mechanism usually includes one top abutment member per lift assembly, a top abutment track carried by a lift assembly, and retraction means capable of positioning an abutment member in register with the abutment track, when necessary.
In addition, the abutment mechanism has a low abutment ring that is common to all of the blades. The low abutment ring is movable in a groove.
In flight, only one blade at a time may potentially move downwards so as to push against the ring. The ring therefore does not impede the flapping of the lift elements.
In contrast, when on the ground, at low speeds of rotation and when stationary, the lift assemblies come simultaneously into contact with the ring, such that the ring prevents all of them from moving in flapping. Under such circumstances and given its function, such a ring is sometimes referred to as a “droop restrainer ring” or “anneau réciproque” in French language.
A known abutment mechanism for a blade thus comprises a low abutment ring common to all of the lift elements, and one retractable high abutment ring for each lift element.
Those two abutment systems are independent and tend to increase the number of mechanical parts needed in a space that is, by its very nature, limited.
Furthermore, on a heavy aircraft the low abutment ring may present weight that is not negligible, and it can generate a degree of unbalance that needs to be taken into consideration.
Furthermore, replacing the abutment mechanism for maintenance purposes can be tedious, sometimes requiring the associated lift assembly to be removed.
Furthermore, the positioning of an abutment mechanism on a rotor can sometimes be difficult. Specifically, a rotor may have numerous other members, leaving little room to install an abutment system.
Documents FR 2 584 997, U.S. Pat. No. 2,719,593, FR 2 551 723, FR 2 447 856, FR 2 735 094, GB 2 073 121, and EP 1 371 554 describe various abutment systems.
Document U.S. Pat. No. 2,717,653 describes a rotor having an oscillating hub hinged to a mast. The hub carries a plurality of lift assemblies.
In addition, the rotor is fitted with an abutment mechanism comprising an annular plate that slides along the mast to constitute a bottom abutment of the hub. Under such circumstances, the plate is controlled by a retraction system comprising flyweights that are movable under the effect of centrifugal force. When the centrifugal force exerted on the flyweight is low, the plate restricts the angular movements of the hub to angular movements that are of small amplitude. Conversely, when the centrifugal force exerted on the flyweights is high, the plate slides and allow the hub to perform angular movements of large amplitude.
Document U.S. Pat. No. 2,717,653 is thus remote from the field of the invention, i.e. the field of handling the flapping motion of lift assemblies.
Document U.S. Pat. No. 2,475,333 describes a rotor having lift assemblies that are connected to a hub by retention and mobility members.
A scissors linkage having two links that are hinged to each other is arranged between firstly a retention and mobility member and secondly the mast. When the scissors linkage is deployed, both links of the linkage are in alignment and block downward flapping of the lift assembly. When the lift assembly flaps upwards, the scissors linkage folds to allow such flapping movement of the lift assembly.
Document U.S. Pat. No. 5,951,252 describes a deployable high abutment having a threaded rod that moves apart the two branches of a scissors linkage. Each branch carries a pad that acts as an abutment surface.
Documents US 2014/262645, EP 0 135 296, and US 2012/256055 describe lead/lag dampers and have no connection with the invention.
Documents U.S. Pat. No. 2,554,774 and GB 2 024 133 are also known.